Assuming you are looking at the same motor from several sources, the difference is just the distribution channel mark-up. Anything labeled "RV" in the distribution network is likley to be lower volume and higher mark-up than, say, a car part or an industrial part. Beyond that, sometimes a mechanically identical part has a different warranty or different labeling, or may come with a different mounting bracket or some other minor difference. If you are replacing a part, you probably don't need the accessory pieces anyway.
If you are talking about different part numbers or different motor ratings or different motor manufacturers, the differences could be those above or much more. Could not guess without having the specifics on each.
Slide-out motors are not unique - they are just a standard motor selected for that application. If you can identify the base part, you don't have to order from the slide out manufacturer or even from RV product channel, so you can usually save big bucks.